The stability of the mouse germ plasm during storage of eight-cell zygotes in liquid nitrogen for periods up to 4 years will be examined. Histocompatibility mutation rates, as detected by the exchange of skin grafts, and skeletal-dimension changes due to accumulated mutations will be compared between mice that have undergone frozen storage and those that have been maintained by conventional breeding. The ability of zygotes from about 40 different inbred strains of mice to survive freezing, storage, thawing, and transplantation to receptive mothers will be determined.